FLOWMETER TYPES AND THEIR PRINCIPLES INTRODUCTION
Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results. With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquids velocity or the change in kinetic energy. !elocity !elocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. "ecause the pipes cross#sectional area is known and remains constant, the average velocity is an indication of the flow rate. $he basic relationship for determining the liquids flow rate in such cases is% &'! where & ' liquid flow through the pipe Orifices are
the most popular liquid flowmeters in use today. n orifice is simply a flat piece of metal with a specific# si*ed hole bored in it. Most orifices are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available. In practice, the orifice plate is installed in the pipe between two flanges. cting cting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. +ressure taps on either side of the plate are used to detect the difference. Maor advantages of orifices are that they have no moving parts and their cost does not increase significantly with pipe si*e. -onical and quadrant orifices are relatively new. $he units were developed primarily to measure liquids with low eynolds numbers. /ssentially constant constant flow coefficients can be maintained at values below 0111. -onical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application. $he segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. $he unit has the ability to measure flows at low eynolds numbers and still maintain the desired square#root relationship. Its design is simple, and there is only one critical dimension the wedge gap. +ressure drop through the unit is only about half that of conventional orifices. Integral wedge assemblies combine the wedge element and pressure taps into a one#piece pipe coupling bolted to a conventional pressure transmitter. transmitter. 2o special piping or fittings are needed to install the device in a pipeline. Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured. ("ack ("ack to Meter $ypes $able) $able )
For many years differential pressure types of f lowmeters have been the most widely applied flow rate measuring device for fluid flows in pipes that require accurate measurement measurement at reasonable cost. Although a number of different types of f low rate-measuring devices devices are now available, the differential pressure type of flowmeter still makes up the largest segment of the total flow measurement market. This type of device has a flow restriction in the line that causes a differential pressure or "head" to be
developed between the two measure-ment locations. ifferential pressure flowmeters are also known as head meters, and, of all the head meters, the orifice flowmeter is the most widely applied device. !talian physicist iovanni #. $enturi %&'()-&*++ in &'' performed the first recorded work that used orifices for the measurement of fluid flows.
ifferential pressure flowmeters have a change in flow cross section that can be described as a restriction placed in the flow line that causes the velocity of the flowing fluid to change. The difference in pressures between the two measurement locations of the flowmeter is the result of the change in the flow velocities. The volume flow rate through the cross-sectional area is given by, .................................................... Q = A v where/ ..................................Q 0 the volumetric flow rate .................................. A = flow in the cross-sectional area ..................................v 0 the average fluid velocity The most commonly applied orifice is a thin, concentric, and flat metal plate with an opening in the plate, installed perpendicular to the flowing stream in a circular conduit or pipe. !n most applications, a sharp edged hole is bored in the center of the orifice plate. As the flowing fluid passes through the orifice, the restriction causes an increase in velocity. A concurrent decrease in pressure occurs as potential energy %static pressure is converted into kinetic energy %velocity. As the fluid leaves the orifice, its velocity decreases and its pressure increases as kinetic energy is converted back into potential energy according to the laws of conservation of energy. 1owever, there is always some permanent pressure loss due to friction, and the loss is a function of the ratio of the diameter of the orifice bore (d) to the pipe diameter % D. This ratio (d/D) is known as beta ratio. For dirty fluid applications, a concentric orifice plate will eventually have impaired performance due to dirt buildup at the plate. !nstead, eccentric or segmental orifice plates are often used. 2easurements are typically less accurate than those obtained from the concentric orifice plate. 3ccentric or segmental orifices are rarely applied in current practice.
Venturi tubes have
the advantage of being able to handle large flow volumes at low pressure drops. venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. s liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions. $he flowmeters have no moving parts. $hey can be installed in large diameter pipes using flanged, welded or threaded#end fittings. 3our or more pressure taps are usually installed with the unit to average the measured pressure. !enturi tubes can be used with most liquids, including those having a high solids content. ( "ack to Meter $ypes $able)
A venturi creates a constriction within a pipe (classically an hourglass shape) that varies the flow characteristics of a fluid (either liquid or gas) travelling through the tube. As the fluid velocity in
the throat is increased there is a consequential drop in pressure. Italian scientist Giovanni B Venturi (1!"#1$%%) was the first to observe this pheno&enon. 'he fact that a pressure drop acco&panies an increased flow velocity is funda&ental to the laws of fluid dyna&ics. wiss &athe&atician *aniel Bernoulli derived the interrelationship between pressure velocity and other physical properties of fluid in 1+$. ,lassically his theore& is used in the design of aircraft wings to create lift fro& the flow of air over the wing profile. 'here are basically two applications for a venturi. By attaching &ano&eters to three sections of the tube the pressure drop can be &easured and the flow rate through the throat calculated. 'his is ter&ed a Venturi -eter. -ore co&&only a venturi can use this negative pressure to draw a second fluid into the pri&ary flow. 'his effect has found &any applications across a range of industries. owever the basic &echanis& has re&ained the sa&e for al&ost %// years. 'hat is until the arrival of Air&aster0 In the &id 1/s *avid 2- hort 3lu&ber 4or&er 4ire#fighter and 5oi ,arp 5eeper brought na6ve logic and lateral thin7ing to bear to funda&entally re#invent the venturi . is award winning invention was subsequently evaluated and qualified by the 'echische 8niversitat a&burg#arburg fluid dyna&ics depart&ent in Ger&any (see 3erfor&ance *ata). 'he patented Air&aster not only delivers i&proved perfor&ance over conventional venturis it can also e9tend its utility by enabling precise ad:ust&ent of the process.
A conventional venturi carries inherent li&itations. 'he constriction strangles the pri&ary flow resulting in bac7pressure that can burden a pu&p with unnecessary load increasing energy costs and shortening its serviceable life. In addition the venturi effect is notorious fic7le fluctuating with slight changes in flow rate te&perature viscosity and other para&eters. 'he Air&aster leaves behind these penalties offering benefits for new and e9isting applications. Bernoulli equation states that the su& of all for&s of energy in a fluid flowing along an enclosed path is the sa&e at any two points in that path (or strea&line). Its for&ulation in the si&plistic hypothesis of inco&pressible flow (fluid &otion with negligible changes in density) is;
where; v ; fluid velocity along the strea&line g ; acceleration of gravity on
h ; height p ; pressure along the strea&line r ; fluid density As a consequence of this law a fluid passing through s&oothly varying constrictions is sub:ected to changes in velocity and pressure. A Venturi is a syste& for speeding the flow of the fluid by constricting it in a cone shape tube. In the restriction the fluid &ust increase its velocity reducing its pressure and producing a partial vacuu&. As the fluid leave the constriction its pressure increase bac7 to the a&bient or pipe level.
4ead more/ http/55www.lenntech.com5venturi.htm6i77+t4l8oqf
